This invention relates to aluminum-based alloys containing lithium as an alloying element. In particular, this invention relates to methods for bonding a lithium-containing aluminum core alloy to a metallic liner material to produce a clad product which can be worked and formed according to conventional aluminum processing techniques with no detriment to appearance or structural integrity.
Alloys of aluminum in which lithium is a major alloying element have the advantage of unusually low density when compared with other aluminum alloys. This has particular value in a wide range of applications, notably in the aircraft industry, where weight reductions readily translate into fuel savings.
Unfortunately, aluminum-lithium alloys are susceptible to fracture upon the application of stress, to a considerably greater extent than other aluminum alloys. To minimize this, lithium concentrations were originally held down to 1.5% or less (by weight). Nevertheless, the problem persisted. Indeed, the low limits on permissible lithium contents limited the percent reduction in density.
Modifications have been made in the alloy composition and processing procedures in attempts to overcome these problems. Typical disclosures are found in Evans et al., European Patent Application Publication No. 88,511 (published Sept. 14, 1983) and its counterpart U.K. Published Patent Application No. 2,115,836A (published Sept. 14, 1983), in which a specified composition range is stated to provide optimum properties including fracture toughness; Peel et al., European Patent Application Publication No. 107,334 (published May 2, 1984) and its counterpart British Patent Application No. 2,127,847A (published Apr. 18, 1984), specifying a composition including zinc for improved properties; Field, European Patent Application Publication No. 90,583 (published Oct. 5, 1983) and its counterpart British Patent Application No. 2,121,822A (published Jan. 4, 1984), which discloses specified homogenization procedures and compositional limitations to dissolve coarse copper-bearing phases in aluminum-lithium-copper-magnesium alloys; and Grimms, British Patent Application No. 2,126,936A (published Apr. 4, 1984) disclosing a technique for superplastic forming.
Some of these disclosures provide for higher lithium concentrations than the original alloys. Unfortunately, high concentrations tend to aggravate the reactivity problems already present at low concentrations. ln particular, the amount of lithium oxide, carbonate and hydroxide formed at the alloy surface as a result of lithium's diffusion to the surface during high temperature processing is increased at high concentrations. These compounds stain the metal surface, detracting from the surface appearance. In addition, the lithium compounds form a fine powder on the surface, which is readily released to the atmosphere upon abrasion to cause a health hazard to workers and other persons in the vicinity. Still further, the oxide, carbonate and hydroxide compounds at the surface seriously interfere with the rolling techniques normally used to form the alloys into the final products. The water or oil used in the rolling process combines with the lithium oxide or carbonate on the surface to form further lithium hydroxide, which fouls the rolling equipment and causes slippage in the roll bite area.
A possible solution to these problems lies in cladding the alloy with a protective metallic liner, which would not only respond to a bright finish treatment for high reflectivity and resist surface oxidation, but would also provide cathodic protection to the core alloy, thus preventing it from exposure to corrosive environments. Cladding is not feasible, however, using conventional techniques. In particular, the elevated temperature usually used in roll bonding processes for cladding aluminum alloys merely heightens the rate of diffusion of lithium from the bulk of the core to the interface, creating lithium-containing compounds there and preventing the formation of a reliable bond.